The goal of this project during the past 10 years has been to increase our understanding of the regulation of plasma lipoprotein (LP) triglyceride (TG) and cholesterol concentrations by studying the flux of plasma LPs containing apolipoprotein B (apoB) in humans. Very low density (VLDL), intermediate density (IDL), and low density lipoproteins (LDL) are heterogeneous. This heterogeneity derives from differences in apolipoproteins and both core and surface lipid composition between subclasses of each major LP class. Studies from a number of laboratories, including ours, have indicated that heterogeneity in VLDL results mainly from secretion of a spectrum of apob-containing LPs by the liver. In contrast, heterogeneity in LDL has been considered to result mostly from the actions of plasma lipases and lipid transfer factors. However, metabolic targeting of specific apob-containing LP populations secreted by the liver could also play an important role in the generation of LDL subclass heterogeneity. In this application we propose to continue to study the determinants of heterogeneity in the apoB-containing LPs, with a particular emphasis on the derivation of LDL heterogeneity. We will conduct in vivo studies of the turnover of apoB-containing LPs utilizing both constant infusions of amino acids labeled with stable isotopes and bolus injections of exogenously radiolabeled LPs. The effects of perturbations known to modify endogenous lipid synthesis by the liver will be investigated. In addition, because recent studies suggest that both the number and physical-chemical characteristics of apoB-containing, LPs secreted by the liver can be affected by the types of remnant LPs taken up by that organ, we will also focus on the relationship between delivery of dietary lipids to the liver and the spectrum of LPs secreted into plasma. The relationship between hepatic uptake/catabolism of plasma LPs, and the assembly/secretion of nascent apoB-containing LPs will also be addressed in vitro, in cultured hepatocytes. In these investigations we will relate post-translational regulation of the assembly and secretion of apoB-containing LPs by HepG2 cells to the intracellular trafficking of endocytosed LPs. The relationship between the effects of different LPs on apob secretion and their effects on cellular cholesterol and triglyceride metabolism will also be determined. Finally, we will attempt to determine if a unique example of apoprotein heterogeneity, one derived from variability in the expression of the apoCIII gene, can affect the metabolism of both apoCIII and apoB.